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LF: Re: Unwanted LF radiation

To: [email protected]
Subject: LF: Re: Unwanted LF radiation
From: "Alan Melia" <[email protected]>
Date: Fri, 10 Jun 2005 15:19:09 +0100
Delivery-date: Fri, 10 Jun 2005 15:19:54 +0100
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Hi Howard, although an ex BT man I dont reckon to know anything about
telephones really !! One thought occurs to me that in the past there have
been problems due to faulty devices in terminals (instruments) radiating
noise and / or oscillations from the lines.

Assuming it is your own drop that is a problem, can you temporarily
disconnect all equipment from your master socket, and then reinstall one at
a time. I am thinking of telephone instrument problems rather than data
units.

If that is not the problem the other area I have found with wideband noise
are fluorescent light particularly when the starter is getting a bit
old.....some still strike after the tube is lit. This of course will not
result in the noise being radiated by the drop.

There is of course a BIG signal located at 100kHz which might be "showing
through the noise" ....Loran-C. There were suggestions that BT would be
starting up an experimental transmission this month but I have seen no signs
of this yet, and I guess you are about as far north in Skipton as I am to
the south east  (Ipswich) of Rugby..... so we should see similar strengths.

It might be easier to locate if there were some recognisable "lines", most
noises I see are eithe 100Hz or 50Hz somewhere and be traced to a number of
known causes. I find ARGO very useful for tracking these using the RX audio.
Both John G4GVC and Peter G3LDO have reported these kinds of noises before,
but with no definitive source found.

Cheers de Alan G3NYK

----- Original Message -----
From: "Howard Aspinall" <[email protected]>
To: <[email protected]>
Sent: 10 June 2005 11:27
Subject: LF: Unwanted LF radiation


This might be just a bit off the beaten track for the reflector, despite
being LF orientated,
but I wonder if anyone has any useful observations or information they
might pass on.
Whilst tracking down some noise interference problems, I discovered
sizeable amounts
of wideband RF between 75 and 130 KHz. This manifests itself as
 "smooth" noise. It extends outside these frequency parameters well into
long/medium
waveband, and is radiating from the overhead BT telephone drop line.
Broadband
internet RF seems the most likely culprit (there being a null between 130
and 140 KHz),
but we do not have any broadband facility here.

An HP spectrum analyser close coupled to the line (common mode) shows this
signal
level to reach at least 5 mV around 100 KHz.

 Tests on BT lines in other areas where broadband is also not installed
show no RF
present other than broadcast stations etc.

 Can anyone say if such common mode RF levels on BT lines can be typical,
or might be
expected where no broadband facilities are connected? Are levels I am
finding here
regarded as acceptable? And does anyone know of any regulations, codes of
practice
or technical standards on the subject?

Howard   G3RXH










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